The present invention relates to a water cooled wall element formed from tubes, which in melting furnaces, preferably electric arc furnaces for melting of steel, replaces completely or in part the casing of refractory material in the region of the furnace above the bath of molten material therein.
The casing of refractory material in the region of arc furnaces above the bath of molten material therein has in the last years increasingly replaced by water cooled wall elements. The advantages of such constructions with respect to forming, the upper part of the furnace from refractory material consists in lowering the operating cost for the furnace as well as in the increase of the useful life of the furnace and the resulting decrease of time for necessary repair.
Since such wall elements formed from water cooled tubes can be dimensioned thinner than the up to now used stones of refractory material, there results also an enlargement of the inner diameter of the furnace in the upper region thereof and therewith also an advantageous increases of the charging volume.
A great number of constructions have in the meantime become known, which are essentially determined by two requirements.
One the one hand, the cooling water should cool all surface regions impinged by heat in a uniform manner, and, on the other hand, these surface regions should be constructed to provide good adhesion for layers of refractory material applied to these surface regions, as well as good adhesion for slag thrown from the molten bath onto the wall element.
The uniform cooling is essential for the useful life of the wall elements since, by not or insufficiently cooled regions and overheating of the same, a premature wear of the element will result. The insulating layer of refractory material, respectively the splattering of slag prevents too high thermic loading of the wall elements, as well as a too high heat transmission to the cooling water.
Requirement for uniform cooling is best accomplished by wall elements which consist of an endless coil of pipe. Such a system is described in the magazine "Fachberichte", volume 2, 1978, on pages 77-82.
This system has the advantage to provide, due to the enforced guiding of the cooling water, a uniform cooling, but this system has the disadvantage that the smooth tube wall provides a poor adhesion for a refractory layer and a splattering of slag. The construction of heat insulating protecting layers is impeded and any protective layers which have been applied peel off easily again and the thus unprotected tube carries off large amounts of heat in an uncontrolled manner to the cooling water.
The known wall elements for box-type design have therefore on the surface directed to the interior of the furnace differently shaped projections or indentations which provide good adhesion for the refractory mass, respectively, the splattering of slag. Elements of this construction are for instance disclosed in the DE OS 25 02 712, DE AS 26 59 827, as well as in DE AS 28 25 536.
The disadvantage of these constructions is the insufficient guiding of the cooling water, respectively, the necessary expense to assure a sufficient guiding of the cooling water.
Combinations of the two constructions are also known in which, for instance, water cooled tubes are inserted in corresponding channels on the rear side of cast metal plates, the front side of which is provided with ribs.
Since the heat transmission from the cast plate to the tubes is relatively poor, these tubes are connected by a heat conductive mass with the plate. These elements are to be used in shaft furnaces with relatively low temperatures.
In another combination, the element comprises a cast iron or copper block and a coiled pipe cast in the block as well as refractory stones, one half of which is cast in the block while the other half protrude beyond the latter. Such an element is expensive to manufacture, especially if the block is formed of copper. Such a copper block has the further disadvantage that it transmits a great amount of heat to the cooling water. If the blcok is formed of cast iron, its durability is limited and after melting of the cast iron body the cooling tube will be exposed, resulting in the disadvantage of a too high heat loss.